The stabbings took place on Wednesday afternoon, Rockford city police said in a statement, adding that a 22-year-old suspect was taken into custody.
The statement did not say anything about a possible motive.
Rockford is located about 90 miles northwest of Chicago.
Those killed included a 15-year-old girl, a 63-year-old woman and two men aged 49 and 22, according to the police.
The police statement said that five people had been wounded, but US media later cited the police saying seven were wounded.
AFP
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